1. Field of the Invention
This invention relates to improved electronic circuits and the methods of manufacture.
2. Description of the Previously Published Art
The manufacture of various types of single and multilayer ceramic electronic packages is generally known. Several methods are embraced, for example, U.S. Pat. No. 3,189,978 teaches a method of screen printing conductive layers of desired patterns on the surfaces of green ceramic sheets which are formed separately. Desired electrical interconnections between layers of green ceramic sheets are formed by punching holes (vias) in the green ceramic sheets, and the respective holes are filled with conductive material. The resultant sheets are stacked into a laminated body so that the conductive layers may have the desired interconnections. The laminated body is sintered to obtain a multilayer structure.
Such prior art used relatively thick green ceramic sheets thus increasing the probability of the holes for interconnections not fully filled with conductive material. This results in the disconnection among conductive layers formed on the different sheets.
In U.S. Pat. No. 3,549,784 a method of screen printing insulative and conductive material alternatively on a ceramic body is described. The problem with this procedure is the deformation of the final ceramic structure because of differences of the shrinkage rates among the ceramic base, the ceramic and the metallic pastes during sintering. In order to avoid such deformation, the art proposes that on the reverse of the ceramic base a metallic layer is to be applied which has an area and thickness similar to the metallic layer or layers printed on the first side of the ceramic base. These additional printing steps result in an increase in manufacturing cost and an increase in the thickness of the final ceramic structure. The resolution and registration of screen printing techniques also limit the number of layers that can be formed.
In U.S. Pat. No. 3,978,248 the ceramic paste and metal paste for screen printing upon the green substrate are developed to exhibit the same shrinkage upon sintering.
Other processes for forming conductive circuit patterns on or within ceramic substrates have been proposed. U.S. Pat. No. 4,540,462 teaches a method wherein a metal sheet is bonded to one or both surfaces of a ceramic sheet. A circuit pattern is formed by coating a photopolymer layer onto the metal layer which is subsequently imaged and developed using conventional imaging and etching techniques. This process is restricted to a maximum of two conductive circuitry layers.
British Pat. No. 1,256,344 teaches a method of making electrical circuits by imaging of a photosensitive composition containing a metal or a heat fusible dielectric on a substrate. The composition is fired after each imaging process to burn off the photosensitive material and to fuse the composition to the substrate. This process has the disadvantage of requiring multiple firings and suffers from distortion due to differential shrinkage between the fired and unfired materials and distortion due to repeated firing.
U.S. Pat. No. 4,336,320 teaches a method for producing cofired conductor and dielectric patterns by depositing a photoresist layer over a dried dielectric thick-film paste. The photoresist is exposed through a photomask and the desired pattern is developed by a process which simultaneously etches the unfired dielectric layer. The resulting voids in the dielectric layer are mechanically filled with a conventional conductive paste not containing photosensitive materials. In another embodiment the dielectric composition contains photosensitive vehicle which can be imaged and developed directly. Again, the resultant voids in the dielectric layer are mechanically filled with a conductive paste not containing photosensitive materials. The patent is dependent upon a stenciling step where the conductive material is pressed or otherwise mechanically filling voids in the dielectric with a conductive paste. Although a key feature is a reduction in the number of firings, this process still requires multiple firings for fabricating multilayer structures. Furthermore, differential sintering shrinkage between the fired and unfired materials could lead to distortions.